Tighten unary op syntax and untupling language

[som-snytt]

Fixes #9241

Disallow unary ops with parens.

Follow up untupling spec where it is not pattern-matching like .tupled. It simply takes the params it knows to take.

[anatoliykmetyuk]

Otherwise LGTM

[som-snytt]

It's not obvious to me that the reason I can't define

def unary_!() = false

is that I wouldn't be able to write prefix op syntax. What if I had no intention of using prefix op syntax? Perhaps my purpose was to sabotage prefix syntax for subclasses.

This is probably the only case where syntax restricts a regular language feature. For example, I can write extensions for StringContext that cannot be used as an interpolator.

[som-snytt]

I used github's "commit suggestion" feature for the first time, but I see that it triggers tests.

[som-snytt]

Rebased for conflicts, but accidentally lost review tweaks added via suggested edits.

Re-added tweaks per review.